1. Field of the Invention
The present invention relates generally to interference cancellation in received wireless communication signals and, more particularly, to forming and using a composite interference signal for interference cancellation.
2. Discussion of the Related Art
In an exemplary wireless multiple-access system, a communication resource is divided into subchannels and allocated to different users. For example, subchannels may include time slots, frequency slots, multiple-access codes, spatio-temporal subchannels, or any combination thereof. A plurality of sub-channel signals received by a wireless terminal (e.g., a subscriber unit or a base station) may correspond to different users and/or different subchannels allocated to a particular user.
If a single transmitter broadcasts different messages to different receivers, such as a base station in a wireless communication system broadcasting to a plurality of mobile terminals, the channel resource is subdivided in order to distinguish between messages intended for each mobile. Thus, each mobile terminal, by knowing its allocated subchannel(s), may decode messages intended for it from the superposition of received signals. Similarly, a base station typically separates signals it receives into subchannels in order to differentiate between users.
In a multipath environment, received signals are superpositions of time delayed (and complex scaled) versions of the transmitted signals. Multipath can cause co-channel and cross-channel interference that correlates the allocated subchannels. For example, co-channel interference may occur when time-delayed reflections of transmitted signals from the same source interfere with each other. Cross-channel interference occurs when signals in a sub channel leak into and, thus, impair acquisition and tracking of other subchannels.
Co-channel and cross-channel interference can degrade communications by causing a receiver to incorrectly decode received transmissions, thus increasing a receiver's error floor. Interference may also have other degrading effects on communications. For example, uncancelled interference may diminish capacity of a communication system, decrease the region of coverage, and/or decrease maximum data rates. Previous interference-cancellation techniques include subtractive and projective interference cancellation, such as disclosed in U.S. Pat. Nos. 6,856,945 and 6,947,474, which are hereby incorporated by reference.